Method of Precision Eye-Tracking Through Use of Iris Edge Based Landmarks in Eye Geometry

ABSTRACT

In the field of eye tracking, greater accuracy or resolution in monitoring movement of the eye can be gained by digitizing the eye, and tracking the movement of a landmark with fixed size and a fixed location relative to the eye&#39;s local coordinate system. The edge of the iris can be used as such a fixed landmark. Through the location and or establishment of at least a portion of the outer edge of the iris and/or an iris center point, both large and small scale eye movements, including but not limited to micro-tremors, can be traced with a higher degree of accuracy. This will aid in the diagnosis of diseases, assessing state of consciousness, and defining brainstem death.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 61/043,349, filed Apr. 8, 2008, entitled “Method ofPrecision Eye-tracking Through Use of Iris Edge Based Landmarks in EyeGeometry”

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains generally to eye tracking technologies,and more particularly to precision eye tracking through use of relativeiris edge based landmarks in eye geometry.

2. Background Information

In addition to the eyes being the “gateway to the soul” (HermanMelville), a subject's eyes are an incredibly sensitive bio-indicatorthat can be utilized for many functions.

Eye movement is the voluntary or involuntary movement of the eyes,helping in acquiring, fixating and tracking visual stimuli. In addition,rapid eye movement (REM) occurs during REM sleep.

“Eye tracking is the process of measuring either the point of gaze(“where we are looking”) or the motion of an eye relative to the head.An eye tracker is a device for measuring eye positions and eyemovements. Eye trackers are used in a wide array of applicationsincluding research in numerous fields, medical diagnosis, psychology, incognitive linguistics and even in advertising and product design. Thereare a number of methods for measuring eye movements. The most popularvariant is a non-invasive technique that uses video images from whichthe eye position is extracted. Other methods use search coils or arebased on the electro-oculogram. The non-invasive technique for recordingeye position relative to the head using a camera to record eye positionrelative to the head is also known as video oculography or VOG. VOGsystems are used by Vestibular Researchers, Ophthalmologist,Otolaryngologists, Physical Therapists, Neurologists, Audiologists,Balance Clinicians, Neurophysiologists, Physiologists, Neuroscientists,Occupational Therapists, and others.

The most widely used current designs are video-based eye trackers. Acamera focuses on one or both eyes and records their movement as thesubject viewer will often look at some kind of stimulus. Most moderneye-trackers use contrast to locate the center of the pupil. Imageprocessing software is utilized to interpret the images to provideobjective data of eye position. This type of image processing softwareis described in “A GEOMETRIC BASIS FOR MEASUREMENT OF THREE-DIMENSIONALEYE POSITION USING IMAGE PROCESSING” Vision Res. Volume 36. No. 3, Mooreet al., pp 445-459, 1996, which is incorporated herein by reference. Ingeneral, most eye tracking devices digitize an image of the, define thepupil using the high contrast difference between the pupil and the restof the eye, and then define the center of the pupil by approximating acircle of the same size or calculating the centroid of the pupil itself.For relatively large-scale eye movements, such as saccades andnystagmus, this method is appropriate despite of the fact that the pupilchanges in size and, to a lesser degree, shape.

Accurate eye position recording and monitoring in three dimensions(3D-yaw, pitch and torsion rotation about line of sight) is asignificant clinical diagnostic tool in the field of vestibulardisorders such as vertigo and other neurological disorders.

There is a need for greater precision in eye tracking systems.

SUMMARY OF THE INVENTION

In the field of eye tracking, greater accuracy or resolution inmonitoring the movement of the eye can be gained by digitizing the eye,and tracking the movement of a landmark with fixed size and a fixedlocation relative to the eye's local coordinate system. At least aportion of the edge of the iris can be used as such a fixed landmark, orsimply landmark. Further, by defining at least a portion of the edge ofthe iris, possibly even calculating its center (or other fixed landmarkrelative to the calculated iris edge), and tracking a portion of theedge and/or the center over time, both large and small scale eyemovements, including but not limited to micro-tremors, can be tracedwith a higher degree of accuracy. This will aid in the diagnosis ofdiseases, assessing state of consciousness, and defining brainstemdeath.

These and other objects of the present invention will be clarified inthe following description which is taken together with the attachedfigures in which like reference numerals represent like elementsthroughout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an image of a subject's eye;

FIG. 2 is a schematic view of an image of the subject's eye of FIG. 1illustrating a representative change in pupil size and shape;

FIG. 3 is a schematic representation of a relative location marker inaccordance with one aspect of the present invention; and

FIG. 4 is a schematic representation of portions of calculating ageometric center of an iris in accordance with one aspect of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

A complete understanding of the invention will be obtained from thefollowing description when taken in connection with the accompanyingdrawing figures, wherein like reference characters identify like partsthroughout. It is also to be understood that the specific devices andprocesses illustrated in the attached drawings, and described in thefollowing specification, are simply exemplary embodiments of theinvention. Hence, specific dimensions and other physical characteristicsrelated to the embodiments disclosed herein are not to be considered aslimiting.

As discussed above, most eye tracking devices digitize an image of the,define the pupil using the high contrast difference between the pupiland the rest of the eye, and then define the center of the pupil byapproximating a circle of the same size or calculating the centroid ofthe pupil itself. For relatively large-scale eye movements, such assaccades and nystagmus, this method is appropriate despite of the factthat the pupil changes in size and, to a lesser degree, shape. However,greater accuracy or even resolution in monitoring the movement of theeye can be gained by digitizing the eye, and tracking the movement of alandmark with fixed size and a fixed location relative to the eye'slocal coordinate system. The edge of the iris can be used as such afixed landmark. The present invention provides that by defining orestablishing at least a portion of the outer edge of the iris both largeand small scale eye movements, including but not limited tomicro-tremors, can be traced with a higher degree of accuracy. This willaid in the diagnosis of diseases, assessing state of consciousness, anddefining brainstem death.

The term landmark within the meaning of the present application willreference a perceptible or visible artifact that is fixed relative tothe local co-ordinate system of the eye. The eye defines a localco-ordinate system, often represented with an axis “through” the centerof the pupil generally along a gaze path and two perpendicular axesthereto. It does not matter where the local co-ordinate system isestablished within the eye, rather that the artifact is fixed in such alocal system such that it is a landmark or a fixed landmark. In otherwords the landmarks will move with the eye.

The concepts of the present invention can be explained in connectionwith FIGS. 1-2. In these figures the wide variation in the pupil size(and shape) will illustrate that the pupil monitoring methods results ina certain amount of error. The pupil center is established typically byapproximating the shape as a circle, and, the center of the circle (theassumed center of the eye), in theory remains unchanged as the circleexpands in diameter. Due to changes in pupil diameter. However these aremerely approximations that lead to noise in the system that prevents thesystem from identifying small eye movements, such as micro-tremors.

The present invention utilized at least a portion of the outer edge ofthe iris as a landmark for tracking eye movement. As shown in FIG. 3,the iris outer segment, namely the lower outer quadrant, can be used asa landmark and this segment viewed to determine eye movement. Therepresentation in FIG. 3 illustrates that the eye position has not movedfrom FIG. 1-2 even though the pupil diameter and shape has changedbetween these two representations.

The image processing for establishing the outer iris edge issubstantially the same as for pupil edge detection, and is the same asgeneral edge detection in image processing as known in the art. The termiris edge is intended to be broad enough to cover various formulationsof finding such an image border. For example, in some edge recognitionprograms, an image will convert the color of different pixels to rangesor even to merely 0 and 1 depending upon a fixed threshold. In order todistinguish between noise and other eye artifacts (eye vessel), the edgedetection algorithm may select find the first 1 in a row and verify thatthe next 2-5 pixels are also 1 before calculating that it is an edge, ormay look at the surrounding X pixel values before calculating that theiris edge has been found. The term iris edge will define the edge of theiris within the scope of image edge detection algorithms.

The present invention contemplates detecting the outer iris edge andutilizing at least a portion of this as a landmark for eye tracking. Thelength of the segment that is utilized can be varied and the quadrantshape is merely an illustration. In some applications a single ordistinct iris edge point will be a sufficient landmark to track. Aplurality of edge points at different locations or of edge segments atdifferent locations could also be utilized as eye landmarks within thescope of the present invention.

A further aspect of the present invention is to utilize the iris edge todefine a center of the iris and to use the center of the iris as an eyetracking landmark.

The center of the iris landmark would be at the centroid of the irisarea. The centroid of an area is very similar to the center of mass of abody. The centroid is calculated using only the geometry of the figure.The general function for calculating the centroid of a geometricallycomplex cross section is most easily applied when the figure is dividedinto known simple geometries and then applying the formula:

$\overset{\_}{x} = \frac{\sum{\overset{\_}{x_{i}}A_{i}}}{\sum A_{i}}$$\overset{\_}{y} = \frac{\sum{\overset{\_}{y_{i}}A_{i}}}{\sum A_{i}}$

The distance from the y-axis to the centroid is x

The distance from the x-axis to the centroid is y

The coordinates of the centroid are ( x, y).

This calculation of a centroid of the iris is represented in FIG. 4 inschematic fashion. One further aspect is that to calculate the iriscentroid the full shape of the iris must be used. Thus the presentinvention contemplates looking for discontinuities along the originallypresumed iris edge, which could be indicative of the lid cutting acrossthe iris portion. In this case the full iris will need to beapproximated between the points of discontinuity based upon the generalshape of the remaining iris portions. The methods of interpolatingbetween the discontinuities based upon the remaining portions of theiris edge are believed to be well understood.

A further iris based landmark would be a centroid of an iris segmentsuch as a quadrant shown in FIG. 3. Calculating the centroid of the pieshaped quadrant follows the same formula above and only needs the irisedge for that segment in order for this point to be calculated.

In summary the key feature of the present invention is the use of theiris edge as a basis for establishing landmarks for eye tracking. Theiris edge may be used itself as a landmark or collection of landmarks inthe form of a single iris edge point, as a single iris edge segment, asa collection of points or segments of the iris edge and combinations ofiris edge points and iris edge segments. Further the iris edge may beused to calculate an iris center point, or other distinct point that isused as a landmark for eye tracking, and these can be used individuallyor in combination with each other. The landmarks that are calculatedfrom the iris edge may be used in combination with the landmarks formedby the iris edge itself.

Further the iris based landmarks (which includes the landmarks formed bythe iris itself) of the present invention can be used with known priorart landmarks to improve the prior art tracking methodologies. Forexample, the present invention can use both pupil center and iris basedlandmarks to track the eye position. It is believed that the iris edgebased landmarks will provide for more precision in eye trackingmethodologies and open up greater diagnostic possibilities accordingly.

Although the present invention has been described above by reference toan embodiment of the invention, the present invention is not limited tothe embodiments described above. Modifications and variations of theembodiment described above will occur to those skilled in the art, inlight of the above teachings without departing from the spirit of thepresent invention The present invention, therefore, to be limited onlyas indicated by the scope of the claims appended hereto and equivalentsthereto.

1. A method of identifying at least one landmark in the eye comprisingthe steps of: digitizing an image of the eye; and identifying at least aportion of the edge of the iris from the digitized image of the eye. 2.The method of claim 1 further including the steps of calculating ageometric center of the iris based upon the edge of the iris.
 3. Amethod of eye tracking comprising the steps of: digitizing an image ofthe eye; identifying at least a portion of the edge of the iris from thedigitized image of the eye; establishing at least one fixed landmarkbased upon the identified portion of the edge of the iris; and trackingthe movement of the at least one landmark over time.
 4. The method ofclaim 3 further including the steps of calculating a geometric center ofthe iris based upon the identified edge of the iris, wherein thegeometric center of the iris is one said landmark.